This invention relates generally to amplifier circuits and more particularly to those amplifier circuits that accept a single input and provide differential outputs (i.e., outputs being 180.degree. out of phase). In radio frequency (RF) applications, such as a balanced mixer, it is often required to drive local oscillator input, RF input or both inputs differentially. It is therefore necessary to reduce phase error between the differential outputs of the amplifier in order to minimize intermodulation distortion. Furthermore, it is important for the amplifier to present a low input impedance for broadband applications as well as ease matching the amplifier input to a desired impedance.
As shown in FIG. 1, a conventional differential amplifier 10 has an input 23 coupled to ground. Conceptually this circuit consists of a common source field effect transistor (FET) 15 and a common gate FET 19. An input signal applied at input 11 appears 180.degree. out-of-phase at output 16 (i.e., the inverting output) and inphase at junction 18. The signal at junction 18 acts as an input to the common gate FET 19, which results in an in-phase signal at an output 20 (i.e., the non-inverting output).
A major disadvantage of the amplifier 10 is that a phase error develops between the two outputs 16 and 20 due to a delay associated with the input signal at junction 13 having to progress through the FET 15 before it reaches the source (i.e., the input) of FET 19. Additionally the amplifier 10 suffers from a narrow operational bandwidth due to its high input impedance. Another disadvantage of the amplifier 10 is the necessity of providing a constant current source 22 to maintain identical amplitudes at outputs 16 and 20.